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What mosfet to use to switch 3.3V power?

Hi, I have an application where I have to switch on/off 2 3.3V low power=20 devices(~50mA max combined) with a 3.3V MCU signal. I am currently=20 using a TP0610K that I have lying around with the following=20 connections. Source - 3.3V Gate - 50K pullup to 3.3V, and MCU control signal Drain - 1M pulldown and device Vcc

This seems to work, but according to the TP0610K datasheet

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3.3V doesn't even begin to bring=20 the fet into saturation mode. Here are the relevant specs of the=20 TP0610K (at least those that I think is relevant): Vgs(th) =3D -1.0/-3.0V min/max Id(on) =3D -50mA (Vgs =3D -4.5V, Vds =3D -10V) Rds(on) =3D 10ohm (Vgs =3D -4.5V, Id =3D -25mA)

*According to the Rds(on)-Vgs graph, Rds(on) rises sharply at about=20 Vgs =3D 5V

Now, while the switch seems to work now, there are a couple of things=20 that worry me.

1) The Id(on) of 50mA is probably lower when Vgs, Vds =3D -3.3V, and=20 this may be a problem since my devices consume about 50mA max 2) Rds(on) rises almost to an asymptote when Vgs < 5V

With that said, I looked at a few other mosfets that I could drop into=20 my existing design, and came up with the following: Fairchild FDV304P - Vth -0.86V, Id -460mA, Rds(on) 1ohm@-3.3V Vgs Fairchild FDV302P - Vth 1V, Id -120mAm Rds(on) ~8ohm@-3.3V Vgs ON NTR1P02LT1 - Vth -1V, Id -1.3A, Rds(on) curve bends at -4V Vgs,=20

0=2E19ohm-2.5V Vgs ON NTR2101P Vth -1V, Id -3.7A, Rds(on) 0.05ohm@-3.3V Vgs ON NTR4101P - Vth -0.72V, Id -3.2A=EF=BC=8C Rds(on) 0.09ohm@-2.5V Vgs Vishay Si2323DS - Vth -1V, Id -4.1A, Rds(on) 0.035ohm@-3.3V Vgs Vishay Si2301BDS - Vth -0.95V, Id -2A?, Rds(on) 0.1ohm@-3.3V Vgs

There are a few more, but I just want to make sure if I have the right=20 idea. Basically what I'm thinking is that I need a mosfet that has the=20 following characteristics:

1) low threshold voltage(all the above have Vgs(th) >=3D -1V 2) Sufficient Id(-120mA and less should be sufficient?) 3) Low Rds(on)(single digit, or even
Reply to
galapogos
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Anyone?

Reply to
galapogos

"galapogos"

** Use a damn BJT - stupid.

...... Phil

Reply to
Phil Allison

1) BJTs have a Vce(sat) voltage drop that I don't want to deal with. Even low Vce(sat) BJTs drop about 0.3V 2) I need it in a SOT-23 package, not many BJTs come in that package 3) afaik mosfets are cheaper?
Reply to
galapogos

I haven't spent a lot of time looking for the perfect part, but you are way off on the saturation drop. Take a look at page 5 of this data sheet:

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Reply to
John Popelish

"galapogos"

Groper Alert !!!

** Bollocks.

The very first one I looked up has a Vce sat typ of * 25mV * at your specified 50 mA Ic.

Makes the " on resistance" = 0.5 ohms !!!!

The value drops at higher current too !!

** See:

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Wake up.

....... Phil

Reply to
Phil Allison

sheet:

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Hmm, I was looking at Fairchild, ON semiconductors and NEC Electronics. I haven't heard of Zetex. ON Semi has 3 new parts that have ultra low Vce(sat), but they're only a few months old. Their old low Vce(sat) parts still have a drop of . Same with NEC's low Vce(sat) parts. Fairchild doesn't even have any low Vce(sat) parts in sot-23 package. Thanks for the heads up though.

Reply to
galapogos

"galapogos"

** My gawd - even a BC327 ( released 35 years ago) has a Vce sat of around 30 mV at 50mA.

........ Phil

Reply to
Phil Allison

A low cost (bout $0.02 in large quantity from Digikey) might be a BC81840. Should do well under .1 volt Vce sat at Ic=.05A with a forced gain of about 20 or 25 (2 mA base current).

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Reply to
jpopelish

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